On page 331 we come to the section on motorcycle crash helmets, as follows PROTECTIVE HEADGEAR Every person driving or riding on a motor bicycle on a road. Must use protective headgear. Must be securely fastened to the head of the wearer by means of straps or other fastening provided for that purpose. Must bear a mark indication compliance with the British Standard OR be of a type which by virtue of its shape, material & construction could be reasonably be expected to afford protection similar to or greater than, a helmet which conforms to the British Standard, an accepted EEA Standard or of ECE regulations.

#2.The Snell Memorial Foundation has developed stricter requirements and testing procedures for motorcycle helmets with racing in mind, as well as helmets for other activities and many riders in America consider Snell certification a benefit when considering buying a helmet while others note that its standards allow for more force (g's) to be transferred to a rider's head than the U.S. Department of Transportation (DOT) standard. However, the DOT standard does not test the chin bar of helmets with them, while the Snell (and ECE) standards do.

In the United Kingdom, the Auto-Cycle Union (ACU) defines a stricter standard for racing than the legal minimum ECE 22.05 specification. Only helmets with an ACU Gold sticker are allowed to be worn in competition, or at track days. Many riders in the UK choose helmets with an ACU Gold sticker for their regular on-road use.

Helmets
I believe that, for many years, the Snell standard has not been sufficient to make it legal for road use in the UK. I'm also fairly sure that the ACU standard is OK.
In the dim-and-distant, the only mark which would suffice was the BSI kitemark, but this has been relaxed in more recent years.